Tornado Warning 27, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
304 PM CDT Wed Jul 12 2023

.MESOSCALE DISCUSSION...
Issued at 300 PM CDT Wed Jul 12 2023

...THREAT FOR STRONG TORNADOES THIS EVENING...

A recent hand surface analysis places a compact meso-low pressure
system centered near Gutenberg, Iowa just north of a surface warm
front stretching from near Dubuque, Iowa southeastward across
Freeport, Aurora, and Valparaiso. Meanwhile, a cold fronts
stretches from near Galena, Illinois to Washington, Iowa. Ahead of
the MCV and south of the warm front, filtered sunshine within an
airmass characterized by pooling surface dew points in the lower
70s is leading to quickly- increasing instability with RAP-
analyzed MLCAPE already exceeding 1000 J/kg ahead of the cold
front. Meanwhile, augmented flow around the periphery of the MCV
is leading to a strongly sheared low- to mid- level kinematic
environment, with both the KLOT/KDVN VWPs showing some 50 kt of
0-6km shear. In fact, locally backed winds along the warm front is
contributing to nearly 40kt of 0-500m shear and 250+ J/kg of
0-500m SRH roughly from Rockford to Romeoville. Such high values
of low-level SRH are relatively rare for our local area for this
time of year.

Over the next few hours, explosive thunderstorm development appears
likely near the Mississippi River with the favorable thermodynamic
and kinematic parameter space supporting a quick evolution into
several supercells. Damaging winds and hail are likely with any
storm. Perhaps more concerning, Bunker's right storm motion along
the warm front will provide unusually long residence times within
the high-magnitude low-level SRH environment, supporting an
unusually high threat for episodic strong tornadoes (EF2+) for our
local area. Extrapolating the location of the warm front, such a
threat appears highest from near Rockford to the Chicago
metropolitan areas between 4 and 8 PM CDT.

Stay weather aware this afternoon and evening. Have multiple ways to
get warnings. And, be ready to take cover in the most
interior, lowest level of your building.

Borchardt

&&

.SHORT TERM...
Issued at 300 PM CDT Wed Jul 12 2023

Through Thursday night...

* Severe weather, including a threat for damaging tornadoes, is
  expected this afternoon through mid evening. Flash flooding is
  also possible, especially in urban areas.

More details on this afternoon and evening's severe weather threat
are addressed in the Mesoscale Discussion above.

Looking ahead to tonight, in the wake of the evening thunderstorms,
residual low-level moisture could result in at least patchy fog
and/or low-stratus development across the area. Due to some
uncertainty with whether this ends up as mainly stratus vs fog,
opted to hold off on a formal gridded forecast mention. A few
light showers and a rumble of thunder will be possible across the
southern fringes of our forecast area toward central Illinois.

Thursday overall looks quiet for most areas. Cannot rule out a few
spotty showers and storms late morning into the afternoon but most
areas should remain dry. Temperatures top out in the lower 80s with
70s expected along the lakeshore.

Petr

&&

.LONG TERM...
Issued at 300 PM CDT Wed Jul 12 2023

Friday through Wednesday...

By the time we get to Friday, our main influencer for our weather
will continue to be a low-pressure system right over the Hudson Bay.
We will also have a little shortwave and potentially, a weak cold
front come through the area. That, along with the low will be our
main forcing mechanisms, and will contribute to more episodes of
potential showers and thunderstorms during the afternoon and evening
hours on Friday. Friday will also be a humid day, due to
temperatures in the lower 80s, and dews getting into the mid 70s.
The NAM also has precipitable water values getting close or up to
the 2 inch margins, so there certainly will be room for high
rainfall rates. The jet stream from this flow pattern is likely
going to stick with us in the near term through mid next week,
which will contribute to the possibility for on and off showers
and thunderstorms.

The low centered over the Hudson Bay will proceed to move downward
over southern Ontario and into the Lake Superior region. The winds
coming from the northwest, forced by the low will help bring in
slightly cooler air and help keep the higher dew points to the
south. Looking at both the GFS and the EURO, it shows that this
ongoing low and shortwave trough should shift to the east,
reducing our chances for rain and lift our region into the ridge,
bringing chances for hot and dry weather toward the middle of
next week.

Bjork/Yack

&&

.AVIATION...
For the 18Z TAFs...

Forecast concerns for the terminals...
*Waves of showers and thunderstorms moving through this afternoon
 with a threat for gusty winds

*Varying wind directions behind storms, but generally favoring a
 southerly direction

*Lowering ceilings overnight into Thursday morning with IFR
 conditions possible

A line of thunderstorms continues to push eastward across
northeast Illinois as of 1730z, but is expected to clear over the
next hour or two. While a brief lull is expected for a couple
hours early this afternoon, guidance continues to indicate that
the atmosphere should destabilize during this period and allow an
additional round of storms to develop in the 21z to 22z timeframe.
If storms are able to develop, they would move east-southeast
across the terminals through 00z this evening and could contain
gusty winds in excess of 30 kts in addition to some large hail.
However, there is some lingering uncertainty in how far south the
storms would propagate with areas along and north of I-80 most
favored at this time. Given that this favored area includes the
terminals I did add a TEMPO in the TAFs for this thunderstorm
threat.

Otherwise, expect winds to generally be from the south with speeds
around 10 kts this afternoon though some variability in
directions can be expected at times. Winds will become northerly
this evening before turning southeasterly for the Chicago
terminals Thursday morning while other sites remain westerly.

Finally, ceilings are expected to lower behind the storms
overnight due to the lingering moisture. Guidance has continued
to indicate that low-end MVFR to possibly IFR conditions are
expected and should linger into Thursday morning. Though it does
look like conditions should start to improve by mid-morning with
VFR conditions forecast to return by the afternoon. There is also
the chance for isolated showers and/or storms Thursday afternoon,
but confidence in coverage is too low for a formal mention at this
time.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch...ILZ013-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108
     until 7 PM Wednesday.

IN...Flood Watch...INZ001 until 7 PM Wednesday.

LM...None.
&&

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
411 PM CDT Wed Jul 12 2023

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-132115-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
411 PM CDT Wed Jul 12 2023 /511 PM EDT Wed Jul 12 2023/

This Hazardous Weather Outlook is for portions of North Central
Illinois...Northeast Illinois and Northwest Indiana.

.DAY ONE...Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
     Elevated Tornado Risk.
     Limited Hail Risk...up to Two Inch Size.
     Limited Damaging Wind Risk...up to 70 mph.
  Elevated Flooding Risk.

DISCUSSION...

Severe thunderstorms are expected this afternoon and evening
across northern Illinois into northwest Indiana. This includes a
threat of damaging tornadoes. Additional hazards include damaging
wind and hail. Localized areas of flash flooding, especially in
urban areas, are also possible.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Thursday...
  Limited Thunderstorm Risk.
Friday...
  Limited Flooding Risk.
  Limited Thunderstorm Risk.
Saturday through Tuesday...
  Limited Thunderstorm Risk.

.SPOTTER INFORMATION STATEMENT...

  Spotters will be needed through this evening.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east southeast at 30 mph.

LMZ740>745-132115-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
411 PM CDT Wed Jul 12 2023

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
     Elevated Waterspout Risk.
     Limited Hail Risk...up to Two Inch Size.
     Limited Thunderstorm Wind Gust Risk...up to 60 kt.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Thursday through Monday...
  Limited Thunderstorm Risk.

Petr

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Preliminary Local Storm Report
National Weather Service Chicago IL
240 PM CDT Wed Jul 12 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0125 PM     Flood            Chicago Ridge           41.70N 87.78W
07/12/2023                   Cook               IL   Public

            A couple inches of standing water in the
            right-hand lanes of I57.

&&

Preliminary Local Storm Report
National Weather Service Chicago IL
247 PM CDT Wed Jul 12 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0238 PM     Rain             3 SSW Burr Ridge        41.71N 87.94W
07/12/2023  M1.69 inch       DuPage             IL   Mesonet

            Mesonet station GW3333 Burr Ridge, 3hr
            total.

&&

Preliminary Local Storm Report
National Weather Service Chicago IL
248 PM CDT Wed Jul 12 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0200 PM     Rain             1 W Lemont              41.67N 88.03W
07/12/2023  M1.36 inch       Will               IL   Mesonet

            Mesonet station LEMI2 Lemont - Des Plaines
            R., 3hr total.

&&

Preliminary Local Storm Report
National Weather Service Chicago IL
249 PM CDT Wed Jul 12 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0224 PM     Rain             1 SE Willow Springs     41.73N 87.85W
07/12/2023  M1.32 inch       Cook               IL   Mesonet

            Mesonet station AT062 Hickory Hills, 3hr
            total.

&&

Preliminary Local Storm Report
National Weather Service Chicago IL
329 PM CDT Wed Jul 12 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0326 PM     Rain             1 NNW Oswego            41.69N 88.36W
07/12/2023  M4.56 inch       Kendall            IL   Trained Spotter

            Spotter measured 4.56in of rain in 24hrs.

&&

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